The conventional Clark oxygen sensor used for continuous in vivo blood gas determination needs frequent recalibration due to membrane fouling, electrolyte thickness changes due to pressure variations, and the fluctuating flow rate of blood. Proof of principle of transient mode of operation was achieved in Phase I. During Phase II a catheter mounted sensor, qualified for animal testing, will be constructed and tested using the above stated principles of construction and operation. Acute ex vivo tests in an extracorporeal loop as well as long term tests of catheter-mounted sensors in arterial sites will be conducted. When developed, the sensor will be used primarily for in vivo monitoring of blood and/or tissue oxygen concentrations. In addition, it is expected that the sensor will also have advantages for in vitro blood gas analyzers.